Why is that fans are understanding that Peeta is not to blame for the effects of his hijacking but don’t extend that understand to Katniss’s mother after Katniss’s father’s death

They are all victims of the world they live in. Of course Peeta isn't to blame for attacking Katniss, we know what happened to him and it wasn't his own free will.

However Mrs Everdeen also was not choosing this. She did not have any treatment accessible, she was not being weak. I am comparing it because just as the outwardly harmful effects of Peeta's hijacking ( his attempts on Katniss's life) are not his fault and he is a victim so is the fact that what was happening to Katniss and Prim with them starving and Katniss having to assume responsibilities in a way no child should have to isn't Mrs Everdeen's fault. If there was treatment and support it would be very different. Katniss's feelings towards her mother are valid but we see her journey of forgiving her mother so I don't understand when readers still hold it against Mrs Everdeen.

We see both characters get better with time and they deserve credit for that, not blame for something they did not choose to endure.